Oil grill with any non-stick spray and place tuna on grill. Grill 1 to 2 minutes then rotate 90 degrees and grill 1 to 2 additional minutes on the same side.
Flip tuna and repeat for a total cook time of 6 to 8 minutes or until internal temperature reaches 130°F to 135°F. Time may vary depending on thickness of tuna. Remove from grill and let rest 5 minutes. Meanwhile, make vinaigrette and assemble salad.
In a blender, add onions, mustard, garlic, honey, vinegar and water. Blend and slowly add olive oil followed by grapeseed oil until an emulsion forms. Season to taste and set aside.
In a large bowl, place power greens, sliced radishes and hearts of palm and toss.
To serve, place a handful of salad on each plate followed by a few slices of tuna and dress with scallion vinaigrette.